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30 U.S. AIR SERVICES August, 1931 These tests go to show that there is little or nothing to be gained electrically by the use of non-conducting material in the aircraft structure where the material becomes wet, as the electrical fields set up are not materially different. A discharge striking conducting material adjacent to inflammable material is likely to set the latter on fire. There are many cases which illustrate this point. Owing to the tendency of a discharge to strike a wet balloon, it was evident that it was necessary to remove the point of contact of any conducting system some distance from the balloon. While this is a distinct disadvantage to racing balloons owing to increased weight, the tests are very interesting as they show the tendency of the wet fabric in diverting a discharge. Figure 7 shows a discharge to ground some few feet distant from a balloon equipped with a cage or lightning rods. In Figure 8 the balloon has been wet. While there was some tendency for the balloon while dry to attract the arc, the point of discharge was too great so that the arc struck to ground. In Figure 8, however, the field set up by the wet balloon was of sufficient magnitude to attract the discharge. The discharge followed the shunt path, then continued from the lower end of the cage to ground. Without the shunt path, the wet balloons were invariably set on fire by the direct stroke. In some cases the fabric seemed to be only slightly damaged. The escaping gas, however, was frequently ignited by the stroke, causing destruction of the balloon unless quickly extinguished. THE PROBABILITY OF A DIRECT HIT IN THE case of free balloons or blimps there would undoubtedly be a considerable difference between the wet and dry conditions in attracting a discharge in the vicinity. In other aircraft as now constructed, however, there would be little different wet or dry. In the tests on models placed near the path of discharge, it was evident that the path of discharge was diverted by an appreciable distance by the presence of the plane. It would seem that the probability of being struck would increase approximately as the square of the greatest linear dimension. The nature of the electrical field, the polarity of the discharge, and the direction of the axis of the aircraft relative to the path of discharge, are all factors which make it difficult to predict the effect of size in increasing the probability of direct hits. Figures 9, 10, 11, and 12 show typical discharges to model planes showing the probable points of contact of the stroke. Figure 13 shows a positive discharge of limited capacity striking a model Zeppelin. The discharge was not sufficient to cause the arc to continue to ground, but illustrates the effect of a large body free of ground. Immediately following the first discharge, another discharge was applied of sufficient magnitude to cause a discharge not only to the model Zeppelin but from the rudder to the ground. It is interesting to note that in all of the tests the fabric used on the Zeppelin and that on the fabric covered duralumin airplane was not ignited by the discharges. This goes to show that the fire hazard is negligible where a path of high electrical conductivity is provided. It would seem that the effective increase in size and the use of metal in the present construction of airplanes should do much to minimize the lightning hazard, even though little or no attention is given to protection. A direct hit to the plane may possibly affect the pilot or passengers in one of the following ways: a. Direct hit. b. By forming a path for the discharge between conducting objects. c. Shock from induced charge. d. Sudden change in air pressure. e. Severe sound or pressure waves. f. Currents induced in the body by an electro-magnetic field. g. Hazard due to the effect of intense light upon the pilot. While the danger from some of these hazards may be absent in many planes, they can be largely reduced if not entirely eliminated in others by proper attention to details of construction, or by applying a protecting scheme which will establish the path of discharge at a distance from the pilot. a. DIRECT HIT IN GENERAL the possibility of a direct hit to the pilot is exceedingly small even in the low wing monoplane with open cockpit. The tests showed that the discharges would enter or leave through the propeller or nose, rudder, wing tips, or landing gear. A lightning rod projecting above and to one side of the pilot would insure the diverting of the stroke even though the pilot's head projected well above the fuselage. In large planes or Zeppelins, the points of contact of the stroke would be considerably removed from the pilot or passengers so that it would appear that the danger from direct stroke may be even less than that in the ordinary dwelling during an electrical storm. b. BY FORMING A PATH FOR THE DISCHARGE BETWEEN CONDUCTING OBJECTS A discharge of lightning may carry a current far in excess of that available in any of the laboratories used for the production of artificial lightning. The fused wire in the basket shown in Figure 2 cannot be duplicated with the heaviest lightning discharges in the laboratory. Records taken at the Forest Rangers' stations show that a stroke of lightning may be sufficient to fuse a No. 14 copper wire. Tests on aerials have shown that wire of larger size is fused, all of which indicate currents exceeding several hundred thousand amperes. The impedance afforded by conductors with this very high rate of discharge will cause a considerable drop in potential. This drop in potential causes the current to divide into multiple paths, the drop in voltage being sufficient to cause the bridging of appreciable air gaps where the impedance is not very large. It is therefore essential that the most careful attention be given to bonding. In order to reduce the impedance it is well to distribute the conductor in several parallel paths. This reduces the reactance and the voltage induced by the high rate of discharge. It is evident that should the pilot come in contact at two points along a conducting member, he is likely to be subject to shock. The thorough bonding, the use of a low impedance multiple path as far away from the pilot as possible, together with a single point of contact with conducting material, will eliminate the danger of shock from drop in potential due to the passage of exceedingly large currents. c. SHOCK FROM INDUCED CHARGE A pilot in an open cockpit may be subjected to an induced charge due to the collapse of the electrostatic field. It would seem that this charge would amount to but little providing the discharge did not cause the pilot to become frightened so as to lose control. The
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